Can the two be used interchangeably?
"As far as I know" means "I'm not 100% sure, but this is the situation as I understand it."
"As far as I'm concerned" means "This is my opinion on the matter. You can accept it or not, but it's how I feel."
The first has to do with a lack of certainty about a fact. The second has to do with an opinion.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.